In order to restore the function of a hip joint deteriorated by disease or injury due to an accident or the like, hip replacement arthroplasty for replacing the hip joint with an artificial hip joint has been carried out conventionally. An artificial hip joint stem (hereinafter generally referred to as “stem”) among members constituting the artificial hip joint is the substantially rod-shaped curved member inserted and fixed to a proximal part of the femoral bone.
The stem is classified into a stem using cement and a stem using no cement (hereinafter generally referred to as “cementless stem”) according to a method of fixing the stem to the femoral bone. As the cementless stem, a bone preservation type stem that reduces injury to the femoral bone and its surrounding tissues has received attention in recent years. Examples of the bone preservation type stem include a type that reduces incisions of the proximal part of the femoral bone, muscles and ligaments by improving insertability of the stem, and a type that reduces the amount of removal of a cancellous bone and reduces injury to bone tissue by devising fixability of the stem.
On the other hand, techniques have been known which improve bonding strength between the stem and the bone by increasing a surface roughness (Ra) in order to roughen the surface of the stem (for example, refer to non patent document 1).
However, an increase in the surface roughness (Ra) of the stem increases friction against the bone. Therefore, when inserting the stem, a distal part of the stem is easy to contact with soft tissue and the cancellous bone at the proximal part of the femoral bone, resulting in poor insertability of the stem. The increase in the surface roughness (Ra) also facilitates bone ongrowth in which bone formation occurs on the surface of the stem, and hence the distal part of the stem is apt to be fixed to the femoral bone. Once the distal part of the stem is fixed to the femoral bone, there is possibility of occurrence of thigh pain and stress shielding in which bone density decreases and the bone shrinks due to less load applied to the bone.